Roland is correct. The Lancaster P.O. doesn't cancel mail anymore. All mail that needs to be canceled is sent to Harrisburg or Allentown. So if you want mail canceled on the day you mail it you have to get it in early on that day.
If you get it to 1400 Harrisburg Pike Blue box (the big blue drive up mail box ) by 500 pm tonight ( i think thats the time for last pick up today ) yes it will be post marked with todays date on it
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